1. Technical Field
The present invention relates generally to lift devices; and, more particularly, to a rotatable tubular metal liftarm assembly suitable for mounting on the deck of a sailboat or similar vessel for on-loading and/or off-loading equipment and supplies. More specifically, the present invention relates to a rotatable tubular metal liftarm assembly which is: i) simple in construction; ii) totally devoid of external supports, braces, pins and similar protuberances; iii) devoid of internal sheaves and/or pulleys; and iv), capable of freely rotating in either direction about its vertical axis through angles of up to 360.degree. without undue twisting of, or otherwise fouling, the load line used to raise/lower objects being on-loaded onto or off-loaded from the vessel.
In its preferred embodiment, the load line is reeved through the tubular liftarm assembly which is devoid of internal sheaves, pulleys or the like; and, consequently, a load carried by the cantilevered free outboard end of the tubular liftarm does not create a rotational force tending to swing the liftarm about its vertical rotational axis, thereby preventing the tendency of heavy loads to inadvertently swing inboard and strike the side of the vessel, its siderail and/or other objects or persons. Moreover, the absence of external supports, braces, pins and/or similar protuberances not only contributes to the smooth, clean, streamlined appearance of the overall device, but, more significantly, enhances the capability of the rotatable liftarm assembly to be freely rotated about its vertical rotational axis during on-loading and/or off-loading operations without risk that the suspended load will be impeded, damaged or will cause damage to the liftarm assembly.
The present invention takes advantage of conventional on-board winches which commonly employ power ratios on the order of 16:1 or more and which are used to pay in or pay out a load line which has its free inboard end wound thereabout during on-loading and/or off-loading operations.
2. Background Art
The prior art is replete with numerous types of liftarm assemblies for sailboats and the like. Typically, such liftarm assemblies employ multi-part block-and-tackle arrangements suspended from the outboard end of a horizontal leg on a fabricated lift-arm assembly comprising aluminum or stainless steel tubing employing external struts, braces and/or pins for reinforcement and/or internal pulleys or sheaves about which the load line is reeved. Such prior art devices are typically quite bulky and capable of only limited rotational movement. Moreover, multi-part block-and-tackle arrangements are disadvantageous because: i) they tend to foul and tangle badly unless kept under tension at all times; ii) to lower a 4:1 tackle from the top of the liftarm to the water level requires paying out four times that distance of line which must be run through the sheave blocks of the tackle arrangement; iii) a heavy consistent downward pull must be maintained on the lower sheave block to overcome all of the friction and line movement in the tackle array; iv) attention must be constantly directed to the inboard end of the tackle system to prevent snags while paying the load line outboard; and v), the operator is required to pull the tackle line inwardly, generally at shoulder level, precisely along the top of the liftarm in order to prevent the tendency of the liftarm to swing inboard and slam the load into the side of the boat or into other objects or persons.
Rohrmann et al U.S. Pat. No. 1,774,996, which issued more than 65 years ago, while not representative of typical liftarm assemblies of the type used on sailboats and like vessels, is of general interest for its early disclosure of a cantilevered type liftarm with an internally reeved load line use to raise and lower the lid of cooking utensils. The device is characterized by the inclusion of a sheave 24 located internally of the horizontal cross arm with the load line being trained about the internal sheave. Such a device is not intended for, nor capable of, transferring heavy loads.
Swiss Pat. No. 384,386 issued to Carrosserie Torsa, Schallbetter & Cie S. A. as the assignee of Rene Salamin, discloses a liftarm assembly similar to that in the Rohrmann et al patent, but which is shown as having utility for raising or lowering small boats to or from a stowed position on top of a car or similar vehicle. As in Rohrmann et al, the tubular liftarm employs internal sheaves or pulleys about which the load line is trained. Additionally, because of the cantilevered construction and the magnitude of the weights to be lifted/lowered, the liftarm is provided with external support struts.
As recently as 1990, Strickland U.S. Pat. No. 4,979,865 issued disclosing a liftarm structure for loading and unloading pick-up trucks wherein the cantilevered liftarm, best shown in Strickland FIG. 4, employs an internal guide pulley 32 and external support struts or legs 27, 40, 41.
Other patents of miscellaneous interest include: i) Beaupre U.S. Pat. No. 4,880,345 [a load hoist assembly for boats]; ii) White U.S. Pat. No. 2,699,203 [an automobile driver's armrest]; iii) Christensen U.S. Pat. No. 3,126,100 [a clamp assembly for attaching a utility tray to a vertical column]; iv) Kindorf et al U.S. Pat. No. 2,616,645 [a pipe hanger]; v) German Pat. No. 130,741 [an internal diametrically extending stiffener member bisecting a tubular element]; and vi), UK Pat. No. 23,790 (1912) [a clip for securing articles to handle-bars on cycles].
Thus, the prior art devices described hereinabove are characterized by their complexity, lack of flexibility, requirements for internal sheaves or pulleys, and requirements for external support struts, braces and/or pins or similar external protrusions which tend to limit free rotation of the liftarm and any supported load about the liftarm's vertical axis. The use of conventional block-and-tackle assemblies of the types commonly employed with such conventional liftarm assemblies is expensive and fraught with difficulties and danger due to the need to maintain the load line under tension at all times so as to prevent tangles and other undue snags. Moreover, conventional block-and-tackle arrangements have limited power ratings, require excessive lengths of load line, and require constant attention by the operator.